Add ability to pass custom args to imagemagick

Hi, 

This is a great module. I started writing my own before finding this and was really pleased that papercut just works straight out of the box.

The only issue I had was in handling uploads from IOS.  Imagemagick was not rotating the image based on the exif information in the uploaded file.

I just added the ability to pass custom arguments to paperclip through the version object like so:

    @version
      custom: ['-auto-orient']

I figured it would be more flexible to add this than to add a custom flag just for what I wanted.

Thanks for the good work.